The present invention relates to a tire for a vehicle wheel in which at least one of the beads has a bead seat with a generatrix whose axially inner end lies on a circle of diameter greater than the diameter of the circle on which the axially outer end lies. This type of design is particularly suitable for the new generations of tires that can be used, within certain limits, under conditions of low pressure, or even zero or almost zero pressure, with no risk of the tire becoming unseated from the rim on which it is mounted. This design is often referred to by the expression “extended mobility”.
For a long time, tire manufacturers have tried to develop a tire avoiding any risk or potential danger in the case of an abnormally low pressure or indeed a complete loss of pressure. One of the difficulties encountered relates to running flat or under very low pressure. This is because, when running with conventional tires at very low pressure, or even zero pressure, there is a very high risk of the beads becoming unseated from the perimeter of the rim against which they would be held by the pressure.
Many solutions have been tested in order to alleviate these drawbacks. Often these solutions create further difficulties as regards mounting the tire on the rim and removing it therefrom.
Moreover, the clamping of the tire onto the rim is an essential function for ensuring the quality of the tire in operation. This is because it has a direct or indirect effect on many aspects, such as mounting or fitting (sometimes called “clipping-in”) of the tire, sealing of the tire, rotation on the rim, etc. These functions are all critical and require specific characteristics and rigorous manufacture of the products, particularly if high quality standards are sought. However, the rims and the tires often have, for the same code, slightly different dimensions, principally due to manufacturing tolerances. These dimensional variations make it more difficult to fulfill the various functions listed above.
To fulfill these functions, two broad categories of solutions are used on an industrial scale. Firstly, in the case of conventional tires, the bead wire provides all these functions simultaneously.
More recently, in several types of products manufactured by the Applicant, the conventional bead wire has been replaced with an anchoring region comprising, in particular, arrangements of circumferential cords cooperating with the carcass-type reinforcement structure via an anchoring or bonding mix. Here again, the anchoring region provides all the abovementioned functions.
In both cases, however, it is difficult to optimize certain parameters, as very often the improvement of one is to the detriment of another. This game of seeking a compromise between advantage on the one hand and disadvantage on the other therefore has certain limits, since it is often difficult to tolerate inferior performance as regards some aspects.
Document EP 0 582 196 discloses a tire having a tread extended by two sidewalls, two beads and a carcass anchored in the two beads with an annular reinforcement. The carcass consists of wires placed adjacently and aligned circumferentially and in contact with at least one layer of bonding rubber of very high elastic modulus in the bead fastening region comprising the annular reinforcement. In this tire, the annular reinforcement of the bead fastening region consists of stacks of circumferential wires with a layer of bonding rubber of very high elastic modulus interposed between the carcass reinforcement wires and these stacks. This embodiment is intended for conventional tires in which the beads are held against the rim gutter owing to the tire inflation pressure. In this type of arrangement, the loads are predominantly of lateral or axial type, inducing high compressive forces acting substantially axially from the walls towards the center of the said bead. These forces increase with the inflation pressure. The increase in pressure tends to make the bead slip against the flange, radially outwards. The radially inwardly induced loads on the rim seat decrease with the increase in pressure, or with any increase in the tension of the carcass-type reinforcement structure.
Moreover, it should be pointed out that the stacks of wires are aligned in a direction approximately parallel to the orientation of the profile of the rim flange against which the bead presses. The profile of the bead of this type of tire is relatively narrow and elongate; the anchoring is distributed over most of the height and width of the bead. Passage of the carcass in the bead is in general approximately central with respect to the walls of the said bead. Moreover, in the case of a relatively narrow bead subjected to predominantly axial loads, neither the inflation pressure nor the tension induced in the carcass allow the generation of large moments or couples, tending to make the bead pivot or rotate on itself.
With such a type of tire, if the pressure drops and running continues, the tire is no longer held against the rim and, in most cases, unseating from the rim occurs.
Document EP 0 673 324 discloses a running assembly comprising at least one tire with radial carcass reinforcement anchored in each bead and a rim of particular shape. This rim has a first seat with a generatrix such that the distance of the axially outer end of the said generatrix from the axis of rotation is less than the distance from its axially inner end and is bounded axially to the outside by a rim flange or projection. The tire has bead seats suitable for mounting on this rim. The type of tire/rim interface proposed in that document has many advantages over the solutions already known, especially as regards ease of mounting/demounting, while still allowing a certain travel despite a drop in pressure.
Document EP 0 748 287 discloses a solution allowing a first optimization of the base technology described in the aforementioned document EP 0 673 324. This relates to a tire in which at least one bead has a structure allowing the clamping of the said bead to be modified according to the tension in the carcass reinforcement, and especially to be strengthened when the inflation pressure increases to its normal value. The document thus proposes the use of a bead with anchoring of the end of the carcass by an upturn of the latter around the base of the bead wire, via the axially and radially inner sides with respect to the bead wire. The bead also has, adjacent to the bead wire and axially to the outside of the latter, a profile or wedge made of a rubber mix having a relatively high hardness against which the bead wire can exert a compressive force when the tension in the carcass reinforcement increases. This compressive force results in self-clamping of the bead toe onto the mounting rim. The tension in the carcass therefore causes the bead wire to be displaced outwards, so that the latter generates the said compressive force. In such a configuration, the presence of a conventional bead wire and the upturn of the carcass under the bead wire are presented as being absolutely essential for generating the compressive force. This limits the possibilities of envisaging other types of arrangement.
Moreover, document EP 0 922 592 discloses two embodiments with anchoring of the carcass through an axially outward upturn of the latter. The first embodiment proposes anchoring the carcass in the bead by a radially outward upturn of the end of the carcass. The upturn is surrounded on either side by two radially superimposed layers of metal wires placed axially side by side and covering substantially the entire axial portion along the bead seat. The layers are arranged so as to be parallel to the bead seat. The types of wire and the corresponding dimensions are very precise. The second solution proposed in that document relates to bead seats with different diameters. Securing of the carcass is also effected in a different way than the first solution. The carcass is firstly subdivided into two radially separate portions level with the bead. Each portion is adjoined with a layer of wires placed radially, each layer being placed radially upwardly against each of the carcass portions. The radially outer carcass portion and the radially inner layer of wires are separated by an elastomer-type insert of high hardness provided in the bead. This insert axially lines the central portion of the bead and rises radially outwards and axially inwards, beyond the radial limit of where the metal wires are present.
Both examples of solutions in document EP 0 922 592 have several drawbacks. Thus, the carcass securing proposed in that document requires the presence of an axially outward upturn of the end portion of the carcass. Moreover, the superimposed layers of wires are placed radially close to the bead seat, to a great extent at a radial position closer to the rotation axis than the top portion of the flange on which the bead bears. Unless highly extensible wires are used, mounting/demounting of the tire is difficult to accomplish, owing to the radially unfavourable position of the wires. It should also be pointed out that the stacks are oriented substantially parallel to the profile of the seat against which the bead bears. According to the second solution, the carcass is subdivided into two portions and an insert of high hardness is needed to separate, on the one hand, the layers of wires and, on the other hand, the two carcass portions. However, the carcass is not anchored in the insert. The shape of the insert described is restricting.
Document WO 01/39999 discloses an extended-mobility tire in which each of the beads has an inverted seat, an anchoring region, a bearing region and a transition region. Each of the regions taken in isolation, and likewise all of the regions, form as it were an internal bead capable of undergoing relative movements such as, for example, angular or rotational movements, with respect to another region, or with respect to a virtual center of pressure CP, or with respect to the rim seat, etc.
Preferably, the said bearing region is substantially elongate. It extends, for example, substantially along the bead seat. Load transfer during rotation of the bottom region from the axially internal portion towards the axially external portion is thus possible, with continuation of bearing against at least one portion of the bead seat. Load transfer ensures self-clamping of the toe of the bead against the rim.